Calcite is a highly abundant mineral in the Earth's crust and occurs as a cement phase in numerous siliciclastic sediments, where it often represents the most reactive component when a fluid percolates through the rock. Hence, the objective of this study is to derive calcite dissolution rates on different scales in a reservoir sandstone using mineral surface experiments combined with vertical scanning interferometry (VSI) and two types of core plug experiments. The 3D geometry of the calcite cement phase inside the rock cores was characterized by X-ray micro-computed tomography (µXCT) and was used to attempt dissolution rate upscaling from the mineral surface to the core scale. Initially (without upscaling), our comparison of the far-from-equilibrium dissolution rates at the mineral surface (µm-mm-scale, low fluid residence time) and the surface normalized dissolution rates obtained from the core experiments (cm-scale, high fluid residence time) revealed differences of 0.5-2 orders of magnitude. The µXCT geometric surface area connected to the open pore space GSA Cc,open considers the fluid accessibility of the heterogeneously distributed calcite cement that can largely vary between individual samples, but greatly affects the effective dissolution rates. Using this parameter to upscale the rates from the µm-to the cm-scale, the deviation of the upscaled total dissolution rates from the measured total dissolution rates was less than one order of magnitude for all investigated rock cores. Thus, GSA Cc,open showed to be reasonably suitable for upscaling the mineral surface rates to the core scale.
Diagenetic illite growth in porous sandstones leads to significant modifications of the initial pore system which result in tight reservoirs. Understanding and quantifying these changes provides insight into the porosity-permeability history of the reservoir and improves predictions on petrophysical behavior. To characterize the various stages of diagenetic alteration, a focused ion beam-scanning electron microscopy (FIB-SEM) study was undertaken on aeolian sandstones from the Bebertal outcrop of the Parchim Formation (Early Permian Upper Rotliegend group). Based on 3D microscopic reconstructions, three different textural types of illite crystals occur, common to many tight Rotliegend sandstones, namely (1) feldspar grain alterations and associated illite meshworks, (2) tangential grain coats, and (3) pore-filling laths and fibers. Reaction textures, pore structure quantifications, and numerical simulations of fluid transport have revealed that different generations of nano-porosity are connected to the diagenetic alteration of feldspars and the authigenic growth of pore-filling illites. The latter leads to the formation of microstructures that range from authigenic compact tangential grain coatings to highly porous, pore-filling structures. K-feldspar replacement and initial grain coatings of illite are composed primarily of disordered 1M d illite whereas the epitaxially grown illite lath-and fiber-shaped crystals occurring as pore-filling structures are of the trans-vacant 1M tv polytype. Although all analyzed 3D structures offer connected pathways, the largest reduction in sandstone permeabil-ity occurred during the initial formation of the tangential illite coatings that sealed altered feldspars and the subsequent growth of pore-filling laths and fibrous illites. Analyses of both illite pore-size and crystallite-size distributions indicate that crystal growth occurred by a continuous nucleation and growth mechanism probably controlled by the multiple influx of potassium-rich fluids during late Triassic and Jurassic times. The detailed insight into the textural varieties of illite crystal growth and its calculated permeabilities provides important constraints for understanding the complexities of fluid-flow in tight reservoir sandstones.
Computer X-ray microtomography (µXCT) represents a powerful tool for investigating the physical properties of porous rocks. While calculated porosities determined by this method typically match experimental measurements, computed permeabilities are often overestimated by more than 1 order of magnitude. This effect increases towards smaller pore sizes, as shown in this study, in which nanostructural features related to clay minerals reduce the permeability of tight reservoir sandstone samples. Focussed ion beam scanning electron microscopy (FIB-SEM) tomography was applied to determine the permeability effects of illites at the nanometre scale, and Navier–Stokes equations were applied to calculate the permeability of these domains. With these data, microporous domains (porous voxels) were defined using microtomography images of a tight reservoir sample. The distribution of these domains could be extrapolated by calibration against size distributions measured in FIB-SEM images. For this, we assumed a mean permeability for the dominant clay mineral (illite) in the rock and assigned it to the microporous domains within the structure. The results prove the applicability of our novel approach by combining FIB-SEM with X-ray tomographic rock core scans to achieve a good correspondence between measured and simulated permeabilities. This methodology results in a more accurate representation of reservoir rock permeability in comparison to that estimated purely based on µXCT images.
Chemical zoning of crystals is often found in nature. Crystal zoning can play a role in a mineral’s thermodynamic stability and in its kinetic response in the presence of fluids. Dissolution experiments at far-from-equilibrium conditions were performed using a sandstone sample containing calcite cement crystal patches. The surface normal retreat of the calcite crystals was obtained by vertical scanning interferometry (VSI) in their natural position in the rock. Dissolution rate maps showed contrasting surface dissolution areas within the crystals, in the same locations where electron microprobe (EMP) maps showed the presence of manganese (Mn) and iron (Fe) substitutions for calcium in the calcite structure. Iron zoning was only identified in combination with manganese. Maximum registered manganese contents were 1.9(9) wt.% and iron were 2(1) wt.%. Manganese zoning of only 0.9(5) wt.% resulted in around 40 % lower dissolution rates than the adjacent pure calcite zones. The combination of both Mn and Fe cation substitutions resulted in one order of magnitude lower dissolution rates compared to pure calcite in the same sample. These results show that mineral zoning can significantly affect reaction rates, a parameter that needs better understanding for the improvement of kinetic geochemical models at the pore scale.
The fluvial-aeolian Upper Rotliegend sandstones from the Bebertal outcrop (Flechtingen High, Germany) are the famous reservoir analog for the deeply buried Upper Rotliegend gas reservoirs of the Southern Permian Basin. While most diagenetic and reservoir quality investigations are conducted on a meter scale, there is an emerging consensus that significant reservoir heterogeneity is inherited from diagenetic complexity at smaller scales. In this study, we utilize information about diagenetic products and processes at the pore- and plug-scale and analyze their impact on the heterogeneity of porosity, permeability, and cement patterns. Eodiagenetic poikilitic calcite cements, illite/iron oxide grain coatings, and the amount of infiltrated clay are responsible for mm- to cm-scale reservoir heterogeneities in the Parchim formation of the Upper Rotliegend sandstones. Using the Petrel E&P software platform, spatial fluctuations and spatial variations of permeability, porosity, and calcite cements are modeled and compared, offering opportunities for predicting small-scale reservoir rock properties based on diagenetic constraints.
We present a new statistical variance approach for characterizing heterogeneities related to pore spaces in reservoir rocks. Laboratory-based computer microtomography data for reservoir sandstone samples were acquired and processed using advanced image segmentation techniques. The samples were processed using a method based on the digital rock physics concept using the high-performance Navier–Stokes flow solver in the GeoDict commercial software package. The digitized structures were subjected to computational fluid dynamic simulations. The effects of structural matrix modifications caused by the precipitation of minerals on the porosity–permeability relationship and the characterization of the representative elementary volume were assessed. The variances of the digital flow fields were compared at the pore scale (6 µm). The algorithm for analysing variance was benchmarked using a synthetic dataset that provided artificial repetitive structural patterns at both low and high resolutions. This gave an estimate of the sensitivity of the proposed algorithm to minor inhomogeneities. Representative elementary volume variance analysis was performed by comparing the correlation coefficients for various pore–grain composition patterns with the variances of simulated mean flow velocities. Probability density functions indicate that the flow velocities and pore space geometries differed greatly for different samples. The normalized probability density functions of the mean flows shifted to higher velocities as the resolution decreased. We found that a representative elementary volume analysis was more reliably achieved by analysing the mean flow velocity variance than by analysing the pore microstructure alone.
Understanding mineral dissolution is relevant for natural and industrial processes that involve the interaction of crystalline solids and fluids. The dissolution of slow dissolving minerals is typically surface controlled as opposed to diffusion/transport controlled. At these conditions, the dissolution rate is no longer constant in time or space, an outcome observed in rate maps and correspondent rate spectra. The contribution and statistical prevalence of different dissolution mechanisms is not known. Aiming to contribute to close this gap, we present a statistical analysis of the variability of calcite dissolution rates at the nano-to micrometer scale. A calcite-cemented sandstone was used to perform flow experiments. Dissolution of the calcite-filled rock pores was measured using vertical scanning interferometry. The resultant types of surface morphologies influenced the outcome of dissolution. We provide a statistical description of these morphologies and show their temporal evolution as an alternative to the lack of rate spatial variability in rate constants. Crystal size impacts dissolution rates most probably due to the contribution of the crystal edges. We propose a new methodology to analyze the highest rates (tales of rate spectra) that represent the formation of deeper etch pits. These results have application to the parametrization and upscaling of geochemical kinetic models, the characterization of industrial solid materials and the fundamental understanding of crystal dissolution.
Heterogeneity of geological materials poses various problems when evaluating reservoir quality and storage potential. We analysed samples of different sedimentary facies of a Rotliegend sandstone from the Flechtingen High (Northern Germany) to determine the influence of depositional environment and diagenetic history on mineralogical composition and its impact on porosity and permeability. We employed high resolution computer tomography (CT) (voxel size: 2.4 µm) and focussed ion beam – scanning electron microscopy (FIB-SEM) (voxel size: 0.01 – 0.03 µm) for direct pore space and mineral distribution analyses with focus on sub-micrometer zones like feldspar cement boundaries and diagenetically grown illite meshwork pore fillings. As shown by , sub-resolution porosity in CT models can drastically influence flow properties. We found that about 20 - 30 % of the segmented initial pore space is rather porous (φ < 100 %) than pure void (φ = 100 %). Thus, we utilized a Navier-Stokes-Brinkmann approach that allows to refine flow properties of observed CT models using void and porous domains. Porous domain properties were derived from Navier-Stokes simulations on FIB-SEM models. For comparison low temperature N2 adsorption and He-porosimetry data were used to characterize bulk permeability (Kbulk), total porosity (φtotal), BET surface area and pore size distributions (PSD). X-ray diffraction (XRD) Rietveld analyses gave quantitative mineralogical information about the different facies and the clay mineral pore fillings. Cross bedded layers show slightly higher Kbulk values (3.8 – 5.1 mD) compared to laminated layers (1.5 – 2.3 mD) which correlates with feldspar cement content. However, calcite and clay cement phases show variance between the samples but no correlation with permeability and porosity. This can be attributed to the pore structure of these cement/grain interfaces with pore radii below 0.05 µm as observed by FIB-SEM. Interfaces connected to feldspar cements show open pore networks with pore radii up to 0.5 µm. Such areas are particularly relevant for quantifying fluid flow as treating these spaces as pure voids, when in fact they represent semi-porous rock, will lead to an overestimation of simulated permeability values (compared to measured Kbulk) by more than one order of magnitude (40 – 50 mD).
Mineral dissolution is crucial for practically all geosciences and many industries including, hydrocarbon exploration and CO2 sequestration. Of particular importance, is the understanding and quantification of mineral dissolution rates and their connection to porosity and permeability evolution in reservoir rocks. Calcite is one of the most abundant and studied minerals on earth, and the most readily dissolved minerals in most natural rocks. We focused on the study of calcite cement dissolution by using an innovative technique based on vertical scanning interferometry (VSI) and Raman spectroscopy that can produce high vertically and laterally resolved topografical data . The calcite cement is directly analysed on natural samples of a Rotliegend sandstone. Detailed petrographic analysis including light microscopy, SEM, EMPA and cathodoluminescence revealed different types of calcite cement (different cation composition). Flow-through experiments were caried out by using a 80 µL fluid-cell for different reaction times at room temperature. VSI topographical data of the samples before and after the experiments were used to contruct reaction rate maps that reveal the bandwith of surface rate variability and its location. Ultimately, the rate maps will complement and extend the less spatially resolved pore scale of techniques such as micro-CT and FIB-SEM.  Fischer C., Arvidson R.S., Lüttge A. (2012), Geochimica et Cosmochimica Acta 98, 177‐185